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          <th width="60%" align="center">Chapter 2. 
                The Db Handle
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            <h2 class="title" style="clear: both"><a id="dbcreate"></a>Db</h2>
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      <pre class="programlisting">#include &lt;db_cxx.h&gt;

class Db { 
public: 
        Db(DbEnv *dbenv, u_int32_t flags); 
        ~Db();

         DB *Db::get_DB(); 
         const DB *Db::get_const_DB() const; 
         static Db *Db::get_Db(DB *db); 
         static const Db *Db::get_const_Db(const DB *db); 
         ... 
}; </pre>
      <p>
        The Db handle is the handle for a Berkeley DB database, which may or may not be part of a
        database environment.
    </p>
      <p>
        Db handles are free-threaded if the 
        <a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a> flag is specified to 
        the <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method when the database is opened or 
        if the database environment in which the database is opened is free-threaded. The handle 
        should not be closed while any other handle that refers to the database is in use; for 
        example, database handles must not be closed while cursor handles into the database remain 
        open, or transactions that include operations on the database have not yet been committed 
        or aborted.  Once the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>, 
        <a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>, 
        <a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>, or
        <a class="xref" href="dbverify.html" title="Db::verify()">Db::verify()</a> 
        methods are called, the handle may not be accessed again, regardless of the method's return.
    </p>
      <p>
        The constructor creates a <a class="link" href="db.html" title="Chapter 2.  The Db Handle">Db</a> object that is the
        handle for a Berkeley DB database. The constructor allocates memory internally; calling
        the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>,
        <a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>, or
        <a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>
        methods will free that memory.        
    </p>
      <p>
        Note that destroying the <a class="link" href="db.html" title="Chapter 2.  The Db Handle">Db</a>
        object is synonomous with calling <code class="literal">Db::close(0)</code>.
    </p>
      <p>
        Each <code class="methodname">Db</code> object has an associated <code class="methodname">DB</code>
        struct, which is used by the underlying implementation of Berkeley DB and its C-language
        API. The <code class="methodname">Db::get_DB()</code>  method returns a pointer to this struct.
        Given a <code class="methodname">const Db</code> object,
        <code class="methodname">Db::get_const_DB()</code> returns a const pointer to the same struct.
    </p>
      <p>
        Given a <code class="literal">DB</code> struct, the <code class="methodname">Db::get_Db()</code> method 
        returns the corresponding <code class="classname">Db</code> object, if there is
        one. If the <code class="literal">DB</code> object was not associated with a 
        <code class="classname">Db</code> (that is, it was not returned from a call
        to the <code class="methodname">Db::get_DB()</code> method), then the result of 
        <code class="methodname">Db::get_Db()</code> is undefined. Given a 
        <code class="literal">const DB struct</code>,
        <code class="methodname">Db::get_const_Db()</code> returns the associated 
        <code class="literal">const Db</code>object, if there is one.
    </p>
      <p>
        These methods may be useful for Berkeley DB applications including both C and C++
        language software. It should not be necessary to use these calls in a purely C++
        application.
    </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
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            <div>
              <h3 class="title"><a id="idm50360"></a>Parameters</h3>
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        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idp88024"></a>dbenv</h4>
              </div>
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          <p>
                 
                <span>If no <span class="bold"><strong>dbenv</strong></span> value is specified,</span> 
                the database is standalone; that is, it is not part of any Berkeley DB
                environment.
            </p>
          <p>
                
                <span>
                    If a <span class="bold"><strong>dbenv</strong></span> value is specified,
                </span>
                the database is created within the specified Berkeley DB environment. 
                The database access methods automatically make calls to the other
                subsystems in Berkeley DB, based on the enclosing environment.  For
                example, if the environment has been configured to use locking, the
                access methods will automatically acquire the correct locks when
                reading and writing pages of the database.
            </p>
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        <div class="sect3" lang="en" xml:lang="en">
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                <h4 class="title"><a id="idm357600"></a>flags</h4>
              </div>
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          </div>
          <p>
                  The <span class="bold"><strong>flags</strong></span> parameter must be set to 0 or the following value:
             </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                <code class="literal">DB_CXX_NO_EXCEPTION</code>
                            </p>
                <p>
                                The Berkeley DB C++ API supports two different error behaviors. By
                                default, whenever an error occurs, an exception is thrown that
                                encapsulates the error information. This generally allows for
                                cleaner logic for transaction processing because a try block can
                                surround a single transaction. However, if this flag is
                                specified, exceptions are not thrown; instead, each individual
                                function returns an error code. 
                            </p>
                <p>
                                If a <span class="bold"><strong>dbenv</strong></span> value is specified,
                                this flag is ignored, and the error behavior of the specified
                                environment is used instead.
                            </p>
              </li>
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              <h3 class="title"><a id="idm1691064"></a>Class</h3>
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        <p>
            <a class="link" href="db.html" title="Chapter 2.  The Db Handle">Db</a>
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      <div class="sect2" lang="en" xml:lang="en">
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              <h3 class="title"><a id="idm1672480"></a>See Also</h3>
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        <p>
            <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 
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